The present invention relates generally to an improved method of manufacturing a composite material. More specifically, the present invention relates to a method of manufacturing a molded material, formed from a base matrix loaded with filler material, that results in a completed composition that has high thermally conductivity and low electrical conductivity.
In the heat sink industries, it has been well known to employ metallic materials for thermal conductivity applications, such as heat dissipation for cooling semiconductor device packages. For these applications, such as heat sinks, the metallic material typically is tooled or machined from bulk metals into the desired configuration. However, such metallic conductive articles are typically very heavy, costly to machine and are susceptible to corrosion. Further, the geometries of machined metallic heat dissipating articles are very limited to the inherent limitations associated with the machining or tooling process. As a result, the requirement of use of metallic materials which are machined into the desired form, place severe limitations on heat sink design particular when it is known that certain geometries, simply by virtue of their design, would realize better efficiency but are not attainable due to the limitations in machining metallic articles.
It is widely known in the prior art that improving the overall geometry of a heat dissipating article can greatly enhance the overall performance of the article even if the material is the same. Therefore, the need for improved heat sink geometries necessitated an alternative to the machining of bulk metallic materials. To meet this need, attempts have been made in the prior art to provide molded compositions that include conductive filler material therein to provide the necessary thermal conductivity. The ability to mold a conductive composite enabled the design of more complex part geometries to realize improved performance of the part.
The attempts in the prior art included the employment of a polymer base matrix loaded with a granular material, such as boron nitride grains. Also, attempts have been made to provide a polymer base matrix loaded with flake-like filler material. These attempts are, indeed, moldable into complex geometries but still do not approach the desired performance levels found in metallic machined parts. In addition, known conductive plastic materials are undesirable because they are typically very expensive to manufacture because they employ very expensive filler materials. Still further, these conductive composite materials must be molded with extreme precision due to concerns of filler alignment during the molding process. Even with precision molding and design, inherent problems of fluid turbulence and collisions with the mold due to complex product geometries make it impossible to position the filler ideally thus causing the composition to perform far less than desirable.
Moreover, the entire matrix of the composition must be satisfactory because heat transfer is a bulk property rather than a direct path property such as the transfer of electricity. A direct path is needed to conduct electricity. However, heat is transferred in bulk where the entire volume of the body is employed for the transfer. Therefore, even if a highly conductive narrow conduit is provided through a much lower conductive body, the heat transfer would not be as good as a body which is consistently marginally conductive throughout the entire body. Therefore, consistency of the thermal conductivity of the entire matrix of the composite body is essential for overall high thermal conductivity.
The fillers used in the prior art to create and enhance the thermally conductive properties of polymer composition also have inherently electrically conductive properties. In general these fillers include aluminum, magnesium or carbon flakes or carbon fibers. By incorporating these fillers into a polymer composition at the generally high ratio required to produce thermal conductivity (as is well known in the prior art), these fillers also impart their inherently electrically conductive properties to the composition. The filler materials used are generally metallic or carbon based and conduct electricity along the same pathways that the heat is conducted. Further, attempting to insulate the composition to prevent electrical conductivity would also interrupt the thermally conductive pathways and defeat the ability of the composition to transfer heat. This electrically conductive property, however, is often undesirable when the thermally conductive polymer is used in electronics applications where transfer of static electrical charges could interfere with the operation of the device or destroy the electronic components therein.
In addition, use of thermally conductive polymer compositions may be indicated in high voltage applications where both the electrically insulative properties of polymer compositions and thermally conductive properties of filled polymer compositions are desirable. For example, use of a lightweight thermally conductive polymer material would be desirable for construction of the outer case for the transformer within a computer power source as an effective solution for dissipating the heat generated during the transformer operation, however, the case must also provide electrical insulation to prevent the transfer of high voltage. In the present state of the art, a technology that provides both thermally conductive and electrically insulative properties is not available.
In view of the foregoing, there is a demand for a composite material that is highly thermally conductive yet electrically insulative. In addition, there is a demand for a composite material that can be molded or cast into complex product geometries. There is also a demand for such a moldable article that exhibits thermal conductivity as close as possible to purely metallic conductive materials while being an effective insulator against transmission of electricity.
The present invention preserves the advantages of prior art thermally conductive plastic compositions namely net-shape moldability and thermal conductivity. In addition, it provides new advantages of electrically insulative properties not found in currently available compositions and overcomes many disadvantages of such currently available compositions.
The invention is generally directed to the novel and unique thermally conductive plastic composite material with electrically insulative properties having particular application in heat dissipation applications where heat must be moved from one region to another to avoid device failure. The composite material of the present invention enables a highly thermally conductive composite material to be manufactured at relatively low cost. The conductive molding composition of the present invention preserves a thermal conductivity above 22 W/mxc2x0K while also providing insulation against electrical conductivity. The thermally conductive composition includes a polymer base matrix of, by volume, between 30 and 60 percent and thermally conductive filler materials, by volume, between 35 and 70 percent.
Prior to mixing the filler material into the polymer base matrix in preparation for injection molding, the filler material is coated with another, thermally conductive and electrically insulative material. The coating material, while highly thermally conductive does not allow electrical conductivity across its surface. As a result, the coating material effectively electrically insulates the filler material and prevents it from conducting electricity throughout the composition while allowing still allowing heat to be transferred through its surface and into the filler. After the coating process, the coated filler is incorporated into the polymer base matrix material and injection molded. Therefore, the present invention preserves the benefits of the thermal conductivity in the prior art, while overcoming the shortfall by insulating against electrical conductivity.
It can be appreciated that the present application has a broad range of applications in areas where use of lightweight material is indicated that can transfer the heat out of an object while preventing the flow of electricity. By way of example, a transformer in a computer power supply is an application that in its operation generates a great deal of heat but must also be insulated to prevent the outflow of voltage. In traditional applications the transformer coils and windings are insulated and supported within a plastic housing with vent holes to allow airflow and passive heat dissipation. Using the present invention the insulation and isolation layers would not be required, as the entire housing would be electrically insulated by the coating material on the conductive filler within the composition. The heat that is generated by the transformer""s operation, however, would be conducted directly through the heat transfer pathways in the thermally conductive filler to the outer surface of the housing and dissipated.
It is therefore an object of the present invention to provide a conductive composite material that has a greater thermal conductivity and lower electrical conductivity than found in prior art composites.
It is an object of the present invention to provide a thermally conductive composite material that does not conduct electricity and is moldable.
It is a further object of the present invention to provide a low cost thermally conductive and electrically insulative composite material.
Another object of the present invention is to provide a thermally conductive composite material that does not conduct electricity and enables the molding of complex part geometries.
It is a further object of the present invention to provide a thermally conductive composite material that does not conduct electricity and is significantly lighter in weight than metallic materials.
It is yet a further object of the present invention to provide a conductive composite material that has a thermal conductivity close to pure metallic materials and does not conduct electricity.